An azole resistant isolate of Candida albicans (Darlington) has been proven to be an erg3 (5,6 desaturase) mutant by showing restoration of ergosterol synthesis following transformation with ERG3 from a wild type strain. The transformant contained six tandem repeats of the ERG3- GAL1 construct and produced twice as much ERG3 transcript as Darlington. Restoring ergosterol synthesis caused a down regulation of ERG1 transcription in Darlington, showing coregulation of ERG3 and ERG11. Sequencing Darling ton ERG3 found allelic dimorphoism. One homologue contained a stop condon in the middle of the open reading frame. The other homologue had two changes in deduced amino acids that were in areas conserved in two other species. Considering that neither homologue produced functional 5,6 desaturase activity, one or both of the altered amino acids may be necessary for enzyme activity. This is the first demonstration that allelic heterozygosity in Candida albicans can differ to this extent at the same locus. The azole resistance of Darlington is not due to the erg3 mutation but is likely due to changes in two deduced amino acids which occur in conserved areas of erg11. Five previously normal patients with cryptococal meningitis were found to have NK cells in their peripheral blood which had a profoundly reduced capacity to kill C.neoformans. This abnormality was found even in patients who were completely recovered and may represent a predisposing factor to infection. Antibody to two different synthetic peptides, chosen from the amino-terminal sequence and from the copper-binding site were found to elicit antibodies which reacted with the full length protein expressed in Saccharomyces cerevisiae.